1. Field of the Invention
The present invention relates to a ladder filter including a series arm resonator and a parallel arm resonator each including an elastic wave resonator.
2. Description of the Related Art
Hitherto, a ladder filter has been used as an RF-stage bandpass filter of a cellular phone or the like. The ladder filter includes a plurality of series arm resonators and a plurality of parallel arm resonators. An elastic wave resonator is generally used as each of the series arm resonators and the parallel arm resonators.
Japanese Unexamined Patent Application Publication No. 2011-101350 discloses an elastic wave resonator utilizing a so-called piston mode. A large-width portion or the like is provided at each end of an interdigital transducer electrode to form a low acoustic velocity region. In an overlap width region, the low acoustic velocity region is provided at each side of a central region of the interdigital transducer electrode. A region between the overlap width region and a busbar is defined as a gap region that is a high acoustic velocity region.
In the ladder filter, a pass band is formed by utilizing the impedance characteristics of the series arm resonators and the parallel arm resonators. However, only by utilizing the resonant frequencies and the anti-resonant frequencies of the series arm resonators and the parallel arm resonators, it is difficult to obtain sufficient out-of-band attenuation characteristics.
Meanwhile, in the elastic wave resonator that is disclosed in Japanese Unexamined Patent Application Publication No. 2011-101350 and utilizes the piston mode, the low acoustic velocity region is provided, and the gap region, which is the high acoustic velocity region, is provided outside the low acoustic velocity region. Accordingly, a transverse mode ripple is suppressed. However, in such a structure, since the low acoustic velocity region has to be provided, there is a problem that the area of each elastic wave resonator increases.